The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube, in which a plurality of regions of a single phosphor screen are dividedly scanned by electron beams emitted from a plurality of electron guns, and a method of manufacturing the same.
These days there is an increasing demand for high-resolution cathode ray tubes having a large screen for high-definition broadcasting, and their screen requires much higher display performance. To meet this demand or requirement, it is essential to make the screen surface flatter and further improve the resolution. At the same time, the screen must be reduced in weight and thickness.
The above demand is met by a cathode ray tube that is described in Jpn. Pat. Appln. KOKAI Publication No. 5-36363. In this device, a plurality of regions of an integral phosphor screen formed on the inner surface of a flat faceplate is dividedly scanned with electron beams that are emitted from electron guns and deflected by means of deflectors.
In this cathode ray tube, a vacuum envelope is formed in a manner such that the flat faceplate and a flat rear plate are opposed to each other with a side wall between them, and a plurality of funnels are bonded to areas around apertures in the rear plate. The integral phosphor screen is formed on the inner surface of the faceplate. A deflector is attached to the outside of each funnel, while an electron gun is arranged in the neck of each funnel.
In the cathode ray tube constructed in this manner, the electron beams emitted from the individual electron guns are deflected by means of magnetic fields that are generated by their corresponding deflectors. The phosphor screen has a plurality of regions, e.g., 20 regions, five in each row in the horizontal direction and four in each column in the vertical direction, and these regions are dividedly scanned with the deflected electron beams. A plurality of divided images formed on the phosphor screen by this divided scanning are connected by means of signals applied to the electron guns and the deflectors, whereupon one large image is formed without any gaps or overlapping on the whole surface of the phosphor screen.
According to the system described above, the cathode ray tube can be reduced in weight and thickness, and its screen surface can be flattened. The reduction in thickness results in a shorter distance between each electron gun and the phosphor screen and facilitates use of an electron lens of lower power. Thus, the diameters of electron beam spots on the phosphor screen are reduced, so that the resolution can be improved.
In the cathode ray tube of this type, moreover, a plurality of columnar support members are arranged between the faceplate and the rear plate, whereby an atmospheric pressure load that acts on the vacuum envelope can be supported. The proximal end of each support member is fixed to the rear plate, while the distal end, wedge-shaped, is in engagement with a black light-absorbing layer of the phosphor screen. When an image is displayed, therefore, the support members can never be seen frontally.
In the cathode ray tube having the aforementioned construction, however, the rear plate, the funnels, and a side plate that constitute a rear envelope cannot be easily positioned with satisfactory accuracy as they are fixed to one another by means of a bonding agent, so that dislocation easily occurs. Accurate relative positioning of the rear plate, funnels, and side wall requires complicated assembling processes, thus entailing an increase in manufacturing cost. Further, joint portions between the individual members lower the reliability of withstand voltage characteristics, vacuum characteristics, etc.
As a measure to solve these problems, a method may possibly be used in which the rear plate, funnels, and side wall are molded integrally from one glass sheet. In this case, the glass sheet as a material is first softened by being heated to a temperature higher than its softening point. Then, the softened glass sheet is held against a carbon mold with a given shape, and is shaped along the mold. Each funnel, made of glass, is reduced in wall thickness on its neck side. A preformed flaring neck is welded to the neck-side end portion of each funnel by burner heating, whereupon the rear envelope is completed.
In the case where the rear envelope is integrally molded in the aforesaid manner, however, glass remains in excess in the corner portions at which the side wall is bent, so that the surplus glass should be driven away to the periphery and cut. This molding operation is very difficult. In addition, the residual glass easily renders the glass thickness distribution uneven, and annealing the glass takes much time.
In the case where the rear envelope is integrally molded, moreover, mold release is difficult due to the difference in thermal expansion coefficient between the glass and the carbon mold. The higher the side wall and the funnels, the more critical this problem will be. Since the rear envelope is integrally molded, it should be regarded as entirely defective if only one of the funnels is cracked or chipped. In welding the necks, furthermore, the whole rear envelope is regarded also as defective if only one of the necks is subject to poor weld. In consequence, the efficiency of manufacture lowers.
Selecting the glass sheet as a material is a problem common to both the integral rear envelope and the rear envelope that is formed by fixing the rear plate, funnels, and side wall by means of a bonding agent. In the cathode ray tube in which the phosphor screen is made to glow with electron beams, as mentioned before, the characteristics of the vacuum envelope, such as volume resistivity, coloring by electron rays, X-ray leakage, etc., should meet their standard requirements. However, there are no existing glass sheets of which all the characteristics meet the requirements.
It is necessary, therefore, to use a surface-treated existing glass sheet or manufacture a novel glass sheet material. However, conventional methods of surface treatment, such as the ion-exchange reinforcement, surface coating, etc., are not effective for the purpose. On the other hand, the manufacture of a novel glass sheet material costs too high to be feasible.